The invention relates to a device for diagnosis a tank ventilation system of a vehicle, including a tank, an adsorption filter that communicates with the tank by way of a tank connection line and has a ventilation line, a tank ventilation valve that communicates with the adsorption filter by way of a valve line, and an on-board pressure source, by means of which the tank ventilation system can be acted upon with a pressure.
The Californian environmental authority (CARB) [California Automotive Repair Bureau] and the American federal environmental authority (Environmental Protection Agency, EPA) require a testing of the functionality of tank ventilation systems in motor vehicles with on-board means (on-board diagnosis, OBDII). In this connection, starting with the model year 2000, leaks of 0.5 mm or greater in size must be detected, signalized, and stored in an on-board memory of the vehicle for later off-board diagnosis.
A process and a device for testing the functionality of a tank ventilation system has been disclosed, for example, by U.S. Pat. No. 5,347,971, in which the tank ventilation system is leak tested by introducing a vacuum into the tank ventilation system and comparing the pressure thus produced with the pressure that is produced by means of a reference leak.
A disadvantage with this process is that the vacuum increases the evaporation of the fuel disposed in the tank.
Therefore DE 195 02 776 C1 has disclosed a device for testing the functionality of a tank ventilation system, in which the testing is carried out by means of an overpressure that is introduced into the tank ventilation system by means of a flow machine. The volume flow introduced is measured by means of a pressure differential measurement at an orifice and then the determination as to whether or not there is a leak is made by means of a comparison with a programmable threshold. With this device, it is problematic that an external shop testing of the tank ventilation system, in which an overpressure is introduced into the tank ventilation system by means of an external pressure source, can only be carried out with difficulty since the tank ventilation system cannot easily be sealed off from the environment and in this respect, a leak can only be detected with difficulty.
A previously unpublished German Patent Application P 196 36 431.0 discloses a process and a device for testing the functionality of a tank ventilation system, which has a tank, an adsorption filter that communicates with the tank by way of a tank connection line and has a ventilation line, a tank ventilation valve that communicates with the adsorption filter by way of a valve line, and an on-board pressure source, by means of which the tank ventilation system can be acted upon with a pressure, in which in order to determine the march of pressure and/or the volume flow supplied, the operating parameters of the pressure source are detected during the introduction of pressure and the existence of a leak is determined from them.
In this device as well, the introduction of an overpressure by means of an external pressure source is problematic since in this device, the adsorption filter cannot be completely sealed off. In a shop test of the tank ventilation system, therefore, air escapes from the tank system and indicates a leak. Consequently one cannot distinguish between a real, actually existing leak and this kind of false leak.